darken method
Implementation
Color darken([double amount = .1]) {
assert(amount >= 0 && amount <= 1, 'Amount must be between 0 and 1');
final hsl = HSLColor.fromColor(this);
final hslDark = hsl.withLightness(
(hsl.lightness - amount).clamp(0.0, 1.0),
);
return hslDark.toColor();
}